1. Field of the Invention
The present invention relates generally to a method and apparatus to compensate for relative vertical displacement between a loading deck and a lifting mechanism such as a crane. More particularly, the invention maintains a lift hook a fixed distance from a deck during loading and unloading operations, even though the deck is moving vertically relative to the lift hook crane assembly.
2. The Prior Art
In the environment of a lift crane mounted on a stationary offshore deck, there has previously existed a problem in loading cargo onto or off of a heaving deck for displacement by a hook and cable associated with the crane. That is, the hook and cable have been maintained substantially in a fixed position, subject only to reel in or pay out by the crane winch. As a result, the heaving deck moves relative to the crane hook, presenting a hazardous and inconvenient condition for loading or unloading. As the deck heaves, the loading crew moves vertically relative to the hook and from their perspective the fixed hook dangles in front of them. Obviously, this presents an inconvenience in attempting to either load or remove cargo from the hook. More important than the convenience factor is that of safety. That is, the loading crew is vulnerable to being struck by the hook and to the possibility of cargo being mishandled to cause injury, which possibility is enhanced by the relative vertical movement between the loading crew and the crane hook.
Other prior art attempts have been made at solving these problems. Such examples are shown in U.S. Pat. Nos. 3,309,065 to Prud'homme and No. 3,662,991 to Lakiza. However, such prior art attempts have not been totally successful in eliminating the problems associated with such a motion compensation method. Neither of these patents has solved the total combination of existing shortcomings, for example in overall product reliability, commercial feasibility, and more importantly, essentially instantaneous response time to the deck heaving action.
Accordingly, these and other shortcomings have previously existed in the prior art.